Since frequencies of millimeter-wave bands exhibit excellent straightness and wide band characteristics as compared with frequencies of micrometer-wave bands, they are in the spotlight in the fields of radars and communication services. In particular, since millimeter-wave frequency bands have small wavelengths, antennas can be easily miniaturized and accordingly, a size of a system can be significantly reduced. Wide band communication using a frequency band of 60 GHz and vehicular radars using a frequency band of 77 GHz have already been commercialized and released as services using such millimeter-wave frequency bands.
As a method of constituting such a millimeter-wave frequency band system, studies on realization of the system in a form of system in packaging (SiP) are being actively conducted to miniaturize a product and reduce costs. A low temperature cofired ceramic (LTCC) or liquid crystal polymer (LCP) technology is considered as one of the most suitable technologies for SiP, and the LTCC or LCP technology basically employs a multilayered substrate and can miniaturize a module and realize low price by embedding passive parts such as a capacitor, an inductor, and a filter in the substrate. Further, since cavities can be formed freely in the multilayered substrate, the degree of freedom in design of the module increases.
Meanwhile, realization of antennas in the SiP system using LTCC is considered as an essential factor in the performance of the system. In general, when a patch antenna operated at a millimeter-wave frequency band, in particular, an ultra-high frequency band of not less than 60 GHz is manufactured, leakage of signals occurs in a form of surface waves flowing along a surface of a dielectric substrate. Such leakage of signals becomes severe as a thickness of the substrate increases, which causes permittivity of the substrates to increase. The leakage of signals reduces a radiation efficiency of the antenna, thus decreasing a gain of the antenna.
The currently released millimeter-wave frequency band modules are manufactured in the form of SiP by using the LTCC technology to reduce size and costs. However, as mentioned above, since permittivity of a ceramic substrate such as LTCC is high as compared with an organic substrate, a radiation efficiency and gain of the antennas decreases when the antenna is formed with a patch antenna. Accordingly, the number of arrays required increases rapidly to achieve a desired antenna gain. Thus, an existing product is manufactured with an organic substrate having low permittivity only for an antenna, and is coupled to an LTCC module in a hybrid form. Due to this, module size and manufacturing costs increase as compared with a case of manufacturing an entire SiP module including an antenna on a single LTCC substrate.